Ultraviolet light sources have long been used for curing photopolymerizable inks on various substrates. However, most conventional types of curing systems develop a significant amount of heat and a build-up of ozone and certain evaporative volatiles in the reaction chamber during the curing operation.
Heat causes a number of problems, such as distortion of printed substrates and misregistration of multicolor. Excessive heat may also result in discoloration, shrinkage, or even blocking of more sensitive inks and substrates. These problems are aggravated when the upward movement of heated air within the curing chamber lifts lighter weight substrates upward within the chamber toward the lamp; fires may result, as well as reduced efficiency may result due to dirt on the lamp. Additionally, excessive heat captured in the curing chamber can greatly reduce the life of the lamp.
Ozone acts as a inhibitor to curing the ink itself. Ozone inhibits the chemical reaction required to polymerize and dry the ultraviolet curing ink. This improper cure can reduce the film strength and its adhesion to the printed substrate.
Numerous methods have been proposed for cooling ultraviolet curing units utilizing combinations of air and water cooling of the lamp itself. However, these types of cooling methods typically sacrifice energy output, and, characteristically, have high maintenance costs. Many ultraviolet curing systems include large housings that attempt to dissipate heat developed inside the reaction chamber. The housings generally include blowers that supply cooling air, which may be directed across the various components of the systems or the substrates themselves. For example, the apparatus disclosed in U.S. Pat. No. 4,434,562 to Bubley et al. includes a blower that produces a cooling air flow directed to the inner and outer surfaces of a reflector shield that houses an elongated curing lamp. While this type of design has been effective in supplying air to various components of the curing unit, it has not been effective for removing the heated air and ozone from the system. Further, it produces excessive air flow across the lamp, which diminishes the ultraviolet radiation output of the lamp.